Non-aqueous electrolyte secondary cell, negative electrode therefor, and method of producing negative electrode

ABSTRACT

Graphite material capable of absorbing and desorbing lithium is employed as the negative electrode material of a non-aqueous electrolyte secondary battery, the negative electrode material being bound by at least one type of material selected from the group consisting of polyethylene, ethylene-vinyl acetate copolymer, ethylene-propylene copolymer, ethylene-propylene-vinyl acetate copolymer, and polypropylene. A non-aqueous electrolyte secondary battery with a high anti-peeling strength of the electrode mix, superiority in the ease of handling, a high reliability in mass production, a superior low-temperature discharge characteristic and cycle characteristic is provided by employing the negative electrode in combination with a rechargeable positive electrode and a non-aqueous liquid electrolyte.

This application is a U.S. National Phase application of PCTInternational application PCT/JP98/05653.

FIELD OF THE INVENTION

The present invention relates to a non-aqueous electrolyte secondarybattery, a negative electrode therefor, and method of manufacturing thenegative electrode.

BACKGROUND OF THE INVENTION

In recent years, non-aqueous electrolyte secondary batteries have beendrawing attention as high output, high energy-density power sources andmany research works are being conducted.

Among the non-aqueous electrolyte secondary batteries, lithium secondarybatteries have heretofore been drawing attention and studied. Lithiumsecondary batteries employ as the positive active material lithiatedtransition metal oxides such as LiCoO₂, LiNiO₂ and chalcogen compoundssuch as MoS₂. These materials have a layer structure in which lithiumions can be reversibly inserted and detached. On the other hand, as thenegative active material, metallic lithium has been employed. However,when metallic lithium is employed in the negative active material,lithium dissolution and deposition reaction is repeated with therepetition of charge and discharge, resulting in the formation ofdendritic lithium on the surface of lithium. The formation of dendriticlithium causes problems of decreasing charge-discharge efficiency and apossible risk of causing short circuit by piercing the separator andgetting in contact with the positive electrode.

In order to solve these problems, lithium alloy plate, metal powders,graphite or other carbon based (amorphous) materials, metal oxides, ormetal sulfides, which can reversibly absorb and desorb lithium are beingstudied as an alternative negative electrode material to metalliclithium.

However, with the use of a lithium alloy plate, there has been a problemthat charge-collecting capability of the alloy decreases with repetitionof deep charge and discharge due to becomins fine of the alloy thuslowering the charge-discharge cycle life characteristic. On the otherhand, when metal powders and powders of carbon materials, metal oxidesor metal sulfides are employed, binders are usually added as anelectrode can not be formed with these materials alone. Regarding carbonmaterials, for example, a method of forming an electrode by adding anelastic rubber-based polymer as the binder is disclosed in JapaneseLaid-Open Patent Application No. Hei 4-255670. With metal oxides andmetal sulfides, an electrically conducting material is also added toincrease conductivity in addition to adding a binder.

When using a carbon material as the negative electrode, the carbonmaterial is usually pulverized into powder and an electrode is formed byusing a binder. When a highly crystalline graphite material is used asthe carbon material, a battery with a higher capacity and higher voltageis obtained compared with a battery using other carbon materials.However, when a graphite material is pulverized, the powder tends toshow flaky configuration. When a negative electrode is formed using thismaterial, as the planar portions of the flaky graphite particles whichare not involved in the insertion-detaching reaction of lithium areoriented in parallel to the plane of the electrode, the high-ratedischarge characteristic declines. Furthermore, when a conventionalrubber-based polymer material is employed as the binder, the bindercovers the graphite particles thus hindering lithium insertion-detachingreaction, drastically lowering the high-rate discharge characteristic ofthe battery, especially the discharge characteristic at lowtemperatures.

Also, as the force of binding with the metallic core material is weak,it is necessary to add a large quantity of the binder, which furtherdeclines the high-rate discharge characteristic. Conversely, when thequantity of addition of the binder is reduced, problems arise such as anincrease in the failure rate due to peeling of the electrode mix in themanufacturing process as the force of binding is weak, or a poorcharge-discharge cycle characteristic due to low resistance to liquidelectrolyte of the rubber-based polymer binder, and a sufficientcharacteristic has not yet been achieved.

Also, during the pressing process of an electrode, there is a problem inthat the graphite particles slide in the direction of pressing thusbreaking bonds of the binder and decreasing the strength of theelectrode.

The present invention addresses these problems and provides batterieshaving a superior high-rate discharge characteristic, especially thedischarge characteristic at low temperatures, and a superiorcharge-discharge cycle characteristic in a large quantity and withstability.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a negative electrodewhich is strong against peeling of the negative electrode mix, superiorin the ease of handling, high in reliability during mass productionprocess, and further, superior in low-temperature dischargecharacteristic and cycle characteristic, and to provide a non-aqueouselectrolyte secondary battery employing the negative electrode.

In accomplishing the object, in a negative electrode for a non-aqueouselectrolyte secondary battery, the negative electrode comprising acarbon material which can reversibly absorb and desorb lithium and abinder, the present invention employs as the binder of the abovenegative electrode material at least one type of material selected fromthe group consisting of polyethylene, polypropylene, ethylene-vinylacetate copolymer, ethylene-propylene copolymer, andethylene-propylene-vinyl acetate copolymer. The present inventionfurther provides a non-aqueous electrolyte secondary battery comprisinga rechargeable positive electrode, a non-aqueous liquid electrolyte, andemploying the above-described negative electrode.

Also, the present invention employs as the binder of the above negativeelectrode material at least one type of material selected from the groupconsisting of polyethylene, polypropylene, polyacrylic acid, acrylate,polymethyl acrylic acid, polymethacrylic acid, methacrylate, andpolymethyl methacrylic acid. The present invention further provides anon-aqueous electrolyte secondary battery comprising a rechargeablepositive electrode and a non-aqueous liquid electrolyte, and employingthe above-described negative electrode.

Further, the present invention employs as the binder of theabove-described negative electrode material at least one type ofmaterial selected from the group consisting of polyethylene,polypropylene, ethylene-acrylic acid copolymer, ethylene-acrylatecopolymer, ethylene-methylacrylic acid copolymer, ethylene-methacrylicacid copolymer, ethylene-methacrylate copolymer, andethylene-methylmethacrylic acid copolymer. The present invention furtherprovides a non-aqueous electrolyte secondary battery comprising arechargeable positive electrode and a non-aqueous liquid electrolyte,and employing the above-described negative electrode.

Also, the present invention employs as the binder of the above-describednegative electrode material at least one type of material selected fromthe group consisting of polyethylene, polypropylene,ethylene-propylene-acrylic acid copolymer, ethylene-propylene-acrylatecopolymer. ethylene-propylene-methylacrylic acid copolymer,ethylene-propylene-methacrylic acid copolymer,ethylene-propylene-methacrylate copolymer, and ethylene-propylene-methylmethacrylic acid copolymer.

The present invention further provides a non-aqueous electrolytesecondary battery comprising a rechargeable positive electrode and anon-aqueous liquid electrolyte, and employing the above-describednegative electrode.

Yet further, the present invention employs as the binder of theabove-described negative electrode material at least one type ofmaterial selected from the group consisting of polyethylene,polypropylene, ethylene-acrylic acid-styrene copolymer,ethylene-acrylate-styrene copolymer, ethylene-methyl acrylicacid-styrene copolymer, ethylene-methacrylic acid-styrene copolymer,ethylene-methacrylate-styrene copolymer, ethylene-methyl methacrylicacid-styrene copolymer, ethylene-propylene-acrylic acid-styrenecopolymer, ethylene-propylene-acrylate-styrene copolymer,ethylene-propylene-methylacrylic acid-styrene copolymer,ethylene-propylene-methacrylic acid-styrene copolymer,ethylene-propylene-methacrylate-styrene copolymer, andethylene-propylene-methyl methacrylic acid-styrene copolymer. Thepresent invention further provides a non-aqueous electrolyte secondarybattery comprising a rechargeable positive electrode and a non-aqueousliquid electrolyte, and employing the above-described negativeelectrode.

In a preferred embodiment of the present invention wherein the negativeelectrode material of a non-aqueous electrolyte secondary batterycomprises a carbon material which is capable of absorbing and desorbinglithium and a binder, the carbon material is high-crystallinity graphiteand at least one type of material selected from the group consisting ofpolyethylene, polypropylene, ethylene-vinyl acetate copolymer,ethylene-propylene copolymer, and ethylene-propylene-vinyl acetatecopolymer is employed as the binder of the negative electrode.

In other preferred embodiment of the present invention, as the binder ofthe negative electrode material at least one type of material selectedfrom the group consisting of polyethylene, polypropylene, polyacrylicacid, acrylate, polymethyl acrylic acid, polymethacrylic acid,methacrylate, and polymethyl methacrylic acid is used. Additionally, bysubstituting a part or the whole of —COOH radical of the acrylic acidand methacrylic acid with —COO⁻Na⁺, K⁺ and the like to obtain acrylateand methacrylate, a negative electrode with a further superior electrodestrength can be obtained.

In a yet other preferred embodiment of the present invention, as thebinder of the negative electrode material, at least one type of materialselected from the group consisting of polyethylene, polypropylene,ethylene-acrylic acid copolymer, ethylene-acrylate copolymer,ethylene-methyl acrylic acid copolymer, ethylene-methacrylic acidcopolymer, ethylene-methacrylate copolymer, and ethylene-methylmethacrylic acid copolymer is used. Additionally, by substituting a partor the whole of the —COOH radical of the acrylic acid and methacrylicacid with —COO⁻Na⁺, K⁺ and the like to obtain acrylate and methacrylate,a negative electrode with a further superior electrode strength can beobtained.

In a still further preferred embodiment of the present invention, as thebinder of the negative electrode material, at least one type of materialselected from the group consisting of polyethylene, polypropylene,ethylene-propylene-acrylic acid copolymer, ethylene-propylene-acrylatecopolymer, ethylene-propylene-methyl acrylic acid copolymer,ethylene-propylene-methacrylic acid copolymer,ethylene-propylene-methacrylate copolymer, and ethylene-propylene-methylmethacrylic acid copolymer is used. Additionally, by substituting a partor the whole of the —COOH radical of the acrylic acid and methacrylicacid with —COO⁻Na⁺, K⁺ and the like to obtain acrylate and methacrylate,a negative electrode with a further superior electrode strength can beobtained.

In a still further preferred embodiment of the present invention, as thebinder of the negative electrode material, at least one type of materialselected from the group consisting of polyethylene, polypropylene,ethylene-acrylic acid-styrene copolymer, ethylene-acrylate-styrenecopolymer, ethylene-methyl acrylic acid-styrene copolymer,ethylene-methacrylic acid-styrene copolymer,ethylene-methacrylate-styrene copolymer, ethylene-methyl methacrylicacid-styrene copolymer, ethylene-propylene-acrylic acid-styrenecopolymer, ethylene-propylene-acrylate-styrene copolymer,ethylene-propylene-methyl acrylic acid-styrene copolymer,ethylene-propylene-methacrylic acid-styrene copolymer,ethylene-propylene-methacrylate-styrene copolymer, andethylene-propylene-methyl methacrylic acid-styrene copolymer is used.Additionally, by substituting a part or the whole of the —COOH radicalof the acrylic acid and methacrylic acid with —COO⁻ Na⁺, K⁺and the liketo obtain acrylate and methacrylate, a negative electrode with a furthersuperior electrode strength can be obtained.

In the present invention, when ethylene-acrylic acid (or acrylate)copolymer, ethylene-methyl acrylic acid copolymer, ethylene-methacrylicacid (or methacrylate) copolymer or ethylene-methyl methacrylic acidcopolymer is employed as the binder, it is preferable to make theethylene content in the range 70%-95%. This is because when the ethylenecontent is less than 70%, the low-temperature discharge characteristicdeclines significantly, and the strength of the electrode decreases whenthe ethylene content exceeds 95%.

The preferred range of the average particle size of the graphitematerial to be used as the negative material of the present invention is5-30 μm. This is because when the average particle size is 5 μm orsmaller, the irreversible capacity of the graphite material increasesthus decreasing the battery capacity, and when the average particle sizeis greater than 30 μm, the low-temperature discharge characteristicdeclines.

Furthermore, the preferred content ratio of the binder to 100 parts byweight of the carbon material is between 0.5 to 8 parts by weight. Thisis because when the content ratio of the binder is below 0.5, sufficientelectrode strength is not obtained whereas the low-temperature dischargecharacteristic declines when the ratio is beyond 8.

Also, the negative electrode of the present invention is rendered moresuperior and desirable in the electrode strength by heat treatment at atemperature between the melting point and the decomposition temperatureof the binder after a mixture of the carbon material and the binder hasbeen coated on a current collector, dried, and pressed, or by pressingat a temperature between the melting point and the decompositiontemperature of the binder. This is because the binder of the negativeelectrode of the present invention melts during pressing or during heattreatment after pressing and solidifies again thus enhancing the bindingproperty. The effect is more pronounced especially when heat treatedduring pressing because of the applied pressure. This effect has notbeen observed with the conventional rubber-based polymers.

In configuring a non-aqueous electrolyte secondary battery employing thenegative electrode of the present invention, lithiated transition metaloxides such as LiCoO₂, LiNiO₂, LiMn₂O₄, etc., can be used as thepositive electrode material. As the liquid electrolyte. a solutionprepared by dissolving an electrolyte salt such as LiPF₅, LiBF₄, etc.,into a mixed solvent of a cyclic carbonate such as ethylene carbonateand a chain carbonate such as ethylmethyl carbonate and the like may beused.

As has been described above, the present invention provides a negativeelectrode which is superior in low-temperature discharge characteristicand in non-peeling strength of the electrode mix and, by using thenegative electrode, it provides a non-aqueous electrolyte secondarybattery which is superior in the ease of handling during massproduction, high in reliability, and superior in dischargecharacteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of a non-aqueous electrolytesecondary battery in an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to a drawing, a description of exemplary embodiments of thepresent invention will be given in the following.

EXAMPLE 1

FIG. 1 shows a vertical cross sectional view of a cylindrical batteryused in the present invention. In the figure, a positive electrode 1 isprepared by mixing LiCoO₂ as the active material and acetylene black asan electrically conducting agent, and additionally,polytetrafluoroethylene as a binder at a weight ratio of 100:3:7, makingpaste by using a thickener, coating the paste on both sides of analuminum foil, drying, and pressing, then cutting to predetermineddimensions (37 mm×350 mm). In addition, an aluminum lead 2 is welded toa positive electrode 1. Negative electrode 3 is prepared by mixing flakygraphite as the carbon material and polyethylene as a binder at apredetermined ratio, coating paste made by using a thickener on bothsides of a copper foil, drying, and pressing, then cutting topredetermined dimensions (39 mm×425 mm). Flaky graphite having averageparticle sizes of 1, 5, 20, 30, and 40 μm was used. The mixing ratios ofpolyethylene as the binder were 0.5, 5, 8, and 10 parts by weightrelative to 100 parts by weight of the carbon material. A nickel lead 4is welded to the negative electrode 3, too. A separator 5 made of amicroporous polyethylene film is interposed between the positiveelectrode 1 and negative electrode 3, all of which are spirally wound toform an electrode group. After disposing insulating plates 6 and 7 madeof polypropylene respectively on the top and bottom ends of theelectrode group, the electrode group is inserted into a case 8 made ofnickel-plated iron. Subsequently, a positive lead 2 and a negative lead4 are respectively welded to a seal plate 9 provided with a safety ventand to the bottom of the case 8. Further, a liquid electrolyte preparedby dissolving lithium hexafluorophosphate as an electrolyte into a 1:3volume ratio mixed solvent of ethylene carbonate and ethylmethylcarbonate to a concentration of 1.5 mol/L is added, sealed with the sealplate 9 with the intervention of a gasket 10 to obtain battery A1 of thepresent invention. Numeral 11 is the positive terminal of the batteryand the case 8 is also serving as the negative terminal. The batterymeasures 17 mm in diameter and 50 mm in height.

The negative electrode was pressed at two temperature points of 25degrees C. and 130 degrees C., and was subsequently dried at 130 degreesC.

EXAMPLE 2

Battery A2 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-vinyl acetatecopolymer as the negative electrode binder.

EXAMPLE 3

Battery A3 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-propylene copolymer asthe negative electrode binder.

EXAMPLE 4

Battery A4 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-propylene-vinylacetate copolymer as the negative electrode binder.

EXAMPLE 5

Battery A5 of the present invention was fabricated in the same manner asin Example 1 with the exception of using polypropylene as the negativeelectrode binder.

EXAMPLE 6

Battery B1 of the present invention was fabricated in the same manner asin Example 1 with the exception of using polyacryl acid as the negativeelectrode binder.

EXAMPLE 7

Battery B2 of the present invention was fabricated in the same manner asin Example 1 with the exception of using polymethyl acrylic acid as thenegative electrode binder.

EXAMPLE 8

Battery B3 of the present invention was fabricated in the same manner asin Example 1 with the exception of using polymethacrylic acid as thenegative electrode binder.

EXAMPLE 9

Battery B4 of the present invention was fabricated in the same manner asin Example 1 with the exception of using polymethyl methacrylic acid asthe negative electrode binder.

EXAMPLE 10

Battery C1 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-acrylic acid copolymeras the negative electrode binder.

EXAMPLE 11

Battery C2 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-methyl acrylic acidcopolymer as the negative electrode binder.

EXAMPLE 12

Battery C3 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-methacrylic acidcopolymer as the negative electrode binder.

EXAMPLE 13

Battery C4 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-methyl methacrylicacid copolymer as the negative electrode binder.

EXAMPLE 14

Battery D1 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-propylene-acrylic acidcopolymer as the negative electrode binder.

EXAMPLE 15

Battery D2 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-propylene methylacrylic acid copolymer as the negative electrode binder.

EXAMPLE 16

Battery D3 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-propylene-methacrylicacid copolymer as the negative electrode binder.

EXAMPLE 17

Battery D4 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-propylene-methylmethacrylic acid copolymer as the negative electrode binder.

EXAMPLE 18

Battery E1 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-acrylic acid-styrenecopolymer as the negative electrode binder.

EXAMPLE 19

Battery E2 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-methyl acrylicacid-styrene copolymer as the negative electrode binder.

EXAMPLE 20

Battery E3 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-methacrylicacid-styrene copolymer as the negative electrode binder.

EXAMPLE 21

Battery E4 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-methyl methacrylicacid-styrene copolymer as the negative electrode binder.

EXAMPLE 22

Battery E5 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-propylene-acrylicacid-styrene copolymer as the negative electrode binder.

EXAMPLE 23

Battery E6 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-propylene-methylacrylic acid-styrene copolymer as the negative electrode binder.

EXAMPLE 24

Battery E7 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-propylene-methacrylicacid-styrene copolymer as the negative electrode binder.

EXAMPLE 25

Battery E8 of the present invention was fabricated in the same manner asin Example 1 with the exception of using ethylene-propylene-methylmethacrylic acid-styrene copolymer as the negative electrode binder.

COMPARATIVE EXAMPLE

Comparative Example battery F of the present invention was fabricated inthe same manner as in Example 1 with the exception of usingstyrene-butadiene copolymer as the negative electrode binder.

Comparison of the low-temperature discharge characteristic, electrodestrength of the negative electrode, and charge-discharge cyclecharacteristic was carried out on the above 26 types of batteries,namely, A1-A5, B1-B4, C1-C4, D1-D4, E1-E8, and F each using a differentnegative electrode binder.

Battery capacity was determined by discharging at a constant dischargecurrent of 180 mA until a discharge termination voltage of 3.0 V isreached after a constant-current constant-voltage charging at a chargingcurrent of 630 mA at a charging voltage of 4.2 V for a charging time of2 hours in a 20 degrees C. environment. The low-temperature dischargecharacteristic was assessed by discharging at a constant dischargecurrent of 900 mA until a discharge termination voltage of 3.0 V isreached after a constant-current constant-voltage charging at a chargingcurrent of 630 mA at a charging voltage of 4.2 V for a charging time of2 hours in a −20 degrees C. environment. Strength of the negativeelectrode was tested by applying 1.5 cm-square cellophane adhesive tapeon the surface of the negative electrode and measuring the forcerequired to peel off the negative electrode mix, which force is thencompared with that of Comparative Example battery F which is defined tobe unity. The relative values thus obtained are shown in Table 1 as theelectrode strength. The larger the electrode strength is, the strongerthe negative electrode mix is against peeling. The charge-dischargecycle test was carried out in a 20 degrees C environment by repeatingconstant-current constant-voltage charging at a charging current of 630mA at a charging voltage of 4.2 V for a charging time of 2 hours andconstant-current discharging at a discharging current of 900 mA until adischarge termination voltage of 3.0 V is reached, and obtaining thenumber of cycles reached until the discharge capacity decreased to 50%of the initial battery capacity.

Table 1 shows the low-temperature discharge capacity, electrodestrength, and charge-discharge cycle characteristic of Example batteriesA1-A5 and Comparative Example battery F. The data is for the case of anaverage particle size of flaky graphite of 20 μm and a binder content of5 parts by weight relative to 100 parts by weight of the carbonmaterial.

TABLE 1 Battery A1 A2 A3 A4 A5 F Rolling Temperature (deg C.) 25 130 25130 25 130 25 130 25 130 25 130 Discharge Capacity at −20 deg C. (mAh)282 355 307 360 273 340 310 355 285 359 44 47 Electrode Strength 1 2 1 21 2 1 2 2 3 1 1 Number of Cycles (cycles) 721 736 508 511 711 720 515522 702 713 447 430

As indicated in Table 1, all of the Example batteries A1-A5 weresuperior to Comparative Example battery F in the low-temperaturedischarge characteristic. This may be attributable to a lower degree ofcarbon particle coverage with the binder compared with ComparativeExample battery F. In other words, the styrene-butadiene copolymer usedin Comparative Example battery F as the binder has a high film-formingability as its glass transition temperature is as low as 0 degree C. orbelow and its particle size is on the order of sub-μm, and, as a result,the binder has a tendency of thinly covering the entire carbon particleeven though the mixing ratio is the same when compared with the negativeelectrode binder of the present invention.

With regard to the electrode strength, all of the Example batteriesA1-A5 of the present invention showed equal or better strength thanComparative Example battery F. Furthermore, in the case pressing wasperformed at 130 degrees C., a negative electrode with a furthersuperior electrode strength was obtained because the binder of thepresent invention melts during pressing and solidifies again under thestate of being pressed.

With regard to the charge-discharge cycle characteristic, Examplebatteries A1-A5 showed a characteristic superior to Comparative Examplebattery F. This may be attributable to superior liquidelectrolyte-resistance of the binder used in these batteries as it doesnot contain double bonds in the primary chain of the polymer and ischemically less reactive with the liquid electrolyte compared with thestyrene-butadiene copolymer binder used in the Comparative Examplebattery F.

Table 2 shows the relationships between the average particle size offlaky graphite and battery capacity and between the average particlesize of flaky graphite and low-temperature discharge characteristic inExample batteries A1-A5 of the present invention and Comparative Examplebattery F. The data is for the case of a binder content of 5 parts byweight to 100 pats by weight of the carbon material. Pressing wascarried out at 25 degrees C.

TABLE 2 Average Particle Size of Battery Discharge Capacity FlakyGraphite Capacity at −20 deg C. Battery (μm) (mAh) (mAh) A1 1 872 321 5920 301 20 932 282 30 938 271 40 943 119 A2 1 852 332 5 910 322 20 925307 30 930 290 40 941 120 A3 1 879 319 5 916 302 20 936 273 30 942 26040 951 111 A4 1 846 329 5 900 319 20 919 310 30 925 286 40 938 119 A5 1876 330 5 919 318 20 939 285 30 942 272 40 949 115 F 1 859 79 5 913 6120 935 44 30 938 20 40 945 3

As can be seen in Table 2, when the average particle size of the flakygraphite is smaller than 5 μm, the battery capacity decreases remarkablyas the irreversible capacity of the carbon material of the negativeelectrode increases, and when greater than 30 μm, the low-temperaturedischarge characteristic declines, suggesting that an average particlesize of flaky graphite in the range 5-30 μm is preferable. Table 3 showsthe relationships between the binder content in parts by weight relativeto 100 parts by weight of the carbon material of the negative electrodeand the low-temperature discharge characteristic and between the bindercontent in parts by weight relative to 100 parts by weight of the carbonmaterial of the negative electrode and the electrode strength of Examplebatteries A1-A5 of the present invention and Comparative Example batteryF. The data is for the case of an average flaky graphite particle sizeof 20 μm. Pressing was carried out at 25 degrees C.

TABLE 3 Discharge Capacity Binder at −20 deg C. Electrode BatteryContent (mAh) Strength A1 0.5 320 <1 5 282 1 8 269 3 10 144 4 A2 0.5 339<1 5 307 1 8 291 3 10 145 3 A3 0.5 315 <1 5 273 1 8 254 3 10 152 4 A40.5 345 <1 5 310 1 8 298 2 10 146 3 A5 0.5 326 1 5 285 2 8 256 4 10 1504 F 0.5 58 =0 5 44 1 8 19 2 10 2 2

It can be seen from Table 3 that when the content in parts by weight ofthe binder relative to 100 parts by weight of the carbon material isgreater than 8 in the Examples of the present invention, thelow-temperature discharge characteristic remarkably declines, and whenit is less than 0.5 the electrode strength decreases not necessarily tozero, resulting in electrode failure such as peeling of the electrodemix. Therefore, it is preferable to make the content of the binder inparts by weight relative to 100 parts by weight of the carbon materialin the range 0.5 to 8.

Additionally, when the temperature of heat treatment after pressing ofthe negative electrode is equal to or below the melting point of thenegative electrode binder, enough electrode strength can not be obtainedbecause the binder does not melt, and at or above the decompositiontemperature of the binder, the binder decomposes and the electrodestrength decreases. As a result, by heat treatment of the negativeelectrode at a temperature between the melting point and thedecomposition temperature of the binder, an electrode with a superiorelectrode strength can be obtained. Same thing is applicable to thetemperature of pressing of the negative electrode.

Though use of one type of binder has been shown in each of the examplesof the present invention, it is apparent that use of a mixture of two ormore types of binder will give similar result.

Table 4 shows the low-temperature discharge characteristic, electrodestrength and charge-discharge cycle characteristic of Example batteriesB1-B4 of the present invention and Comparative Example battery F. Thedata is for the case of an average flaky graphite particle size of 20 μmand the binder content of 5 parts by weight relative to 100 parts byweight of the carbon material.

TABLE 4 Battery B1 B2 B3 B4 F Rolling Temperature (deg C.) 25 130 25 13025 130 25 130 25 130 Discharge Capacity at −20 deg C. (mAh) 105 143 110147 108 146 106 150 44 47 Electrode Strength 5 6 5 6 5 6 5 6 1 1 Numberof Cycles (cycles) 508 515 550 559 543 553 526 531 447 430

As shown in Table 4, all of Example batteries B1-B4 of the presentinvention were superior to Comparative Example battery F in thelow-temperature discharge characteristic. This is considered to be dueto a lower degree of carbon particle coverage with the binder comparedwith Comparative Battery F.

With regard to the electrode strength, too, all of Example batteriesB1-B4 of the present invention were superior to Comparative Examplebattery F. Furthermore, in the case pressing was performed at 130degrees C., a negative electrode with a further superior electrodestrength was obtained because the binder melts during pressing andsolidifies again under the state of being pressed. Also, the reason whythe negative electrode of Example batteries B1-B4 of the presentinvention showed especially high values of strength is considered to bedue to the fact that the negative electrode has a highly polar radical,—COOH or —COOCH₃, and has hence an enhanced adhesiveness with the metalcurrent collector. Furthermore, it was confirmed that when a part or thewhole of —COOH or —COOCH₃ radical is substituted with —COO⁻Na⁺, K⁺ tomake acrylate and methacrylate, adhesiveness with the core material isenhanced.

Example batteries of the present invention also showed acharge-discharge cycle characteristic which is superior to theComparative Example battery F. This is considered to be due to the factthat the binder of these batteries does not have double bonds in theprimary chains of the polymer and is chemically less reactive to liquidelectrolyte thus superior in resistance to liquid electrolyte comparedwith styrene-butadiene copolymer of the used in Comparative Examplebattery F.

Table 5 shows the relationships between the average particle size offlaky graphite and battery capacity and between the average particlesize of flaky graphite and low-temperature discharge characteristic inExample batteries B1-B4 of the present invention and Comparative Examplebattery F. The data is for the case of a binder contents of 5 parts byweight relative to 100 parts by weight of the carbon material. Pressingwas carried out at 25 degrees C.

TABLE 5 Average Particle Discharge Size of Battery Capacity at FlakyGraphite Capacity −20 deg C. Battery (μm) (mAh) (mAh) B1 1 868 210 5 908155 20 924 105 30 931 95 40 940 30 B2 1 871 205 5 911 150 20 926 110 30933 95 40 943 35 B3 1 869 208 5 909 149 20 924 108 30 932 93 40 941 33B4 1 866 211 5 904 153 20 919 106 30 926 91 40 938 32 F 1 859 79 5 91361 20 935 44 30 938 20 40 945 3

As can be seen from Table 5, when the average particle size of the flakygraphite is smaller than 5 μm, the battery capacity decreasessignificantly as the irreversible capacity of the negative electrodecarbon material increases, and when greater than 30 μm, thelow-temperature discharge characteristic declines, suggesting that anaverage particle size range of 5-30 μm of the flaky graphite ispreferable.

Table 6 shows the relationships between the binder content in parts byweight in the negative electrode relative to 100 parts by weight of thecarbon material and the low-temperature discharge characteristic andbetween the binder content in parts by weight in the negative electroderelative to 100 parts by weight of the carbon material and the electrodestrength of Example batteries B1-B4 of the present invention andComparative Example battery F. The data is for the case of an averageflaky graphite particle size of 20 μm. Pressing was carried out at 25degrees C.

TABLE 6 Discharge Capacity Binder at −20 deg C. Electrode BatteryContent (mAh) Strength B1 0.5 175 3 5 105 5 8 90 7 10 38 10 B2 0.5 180 35 110 5 8 96 8 10 37 10 B3 0.5 178 3 5 108 5 8 94 7 10 37 10 B4 0.5 1753 5 106 5 8 90 8 10 34 10 F 0.5 58 =0 5 44 1 8 19 2 10 2 2

As can be seen from Table 6, when the binder content in parts by weightrelative to 100 parts by weight of the carbon material was larger than8, a significant decline in the low-temperature discharge characteristicwas observed, and at 0.5, there was a decrease in the electrodestrength. Therefore, the preferable range of the binder content in partsby weight relative to 100 parts by weight of the carbon material is0.5-8.

Now, with regard to the temperature of heat treatment of the negativeelectrode after pressing, enough electrode strength is not obtained ator below the melting point of the negative electrode binder as thebinder does not melt, and the electrode strength decreases at or abovethe decomposition temperature of the binder as the binder decomposes.Therefore, an electrode with a superior electrode strength can beobtained by heat treatment at a temperature between the melting pointand decomposition temperature of the binder. Same thing applies to thepressing temperature of the negative electrode.

Though a description has been made of use of one type of binder in eachExample of the present invention, it is obvious that similar result willbe obtained by using a mixture of two or more types. It is also obviousthat similar result will be obtained when the binder is used blendedwith polyethylene and polypropylene.

Table 7 shows the low-temperature discharge characteristic, electrodestrength, and charge-discharge cycle characteristic of the Examplebatteries C1-C4 of the present invention and Comparative battery F. Thedata is for the case of an average flaky graphite particle size of 20 μmand the binder content of 5 parts by weight relative to 100 parts byweight of the carbon material.

TABLE 7 Battery C1 C2 C3 C4 F Rolling Temperature (deg C.) 25 130 25 13025 130 25 130 25 130 Discharge Capacity at −20 deg C. (mAh) 170 204 185225 170 200 180 223 44 47 Electrode Strength 3 4 4 5 4 5 4 4 1 1 Numberof Cycles (cycles) 516 527 530 540 522 531 521 539 447 430

As shown in Table 7, all of Example batteries C1-C4 of the presentinvention exhibited a characteristic superior to Comparative Examplebattery F in the low-temperature discharge characteristic. This isconsidered to be due to a lower degree of coverage of the carbonparticles with the binder compared with Comparative Example battery F.

With regard to the electrode strength, too, all of Example batteriesC1-C4 of the present invention were superior to Comparative Examplebattery F. Furthermore, in the case pressing was performed at 130degrees C., a negative electrode with a further superior electrodestrength was obtained because the negative binder of the presentinvention melts during pressing and solidifies again under the state ofbeing pressed thus enhancing the binding property. Also, the reason whythe negative electrode of Example batteries C1-C4 of the presentinvention showed especially high values of strength is considered to bedue to the fact that the negative electrode has a highly polar radical,—COOH or —COOCH₃. Furthermore, it was confirmed that when a part or thewhole of —COOH or —COOCH₃ radical is substituted with —COO⁻Na⁺, K⁺ tomake acrylate and methacrylate, adhesiveness with the core material isfurther enhanced.

Example batteries of the present invention also showed acharge-discharge cycle characteristic which is superior to ComparativeExample battery F. This is considered to be due to the fact that thebinder of these batteries does not have double bonds in the primarychains of the polymer and is chemically less reactive to liquidelectrolyte thus superior in resistance compared with styrene-butadienecopolymer of the binder used in Comparative Example battery F.

Table 8 shows the low-temperature discharge characteristic and electrodestrength for various ethylene contents of the ethylene-acrylic acidcopolymer in Example batteries C1-C4 of the present invention. The datais for the case of an average particle size of 20 μm of the flakygraphite and a binder content of 5 parts by weight relative to 100 partsby weight of the carbon material. Pressing was carried out at 25 degreesC.

TABLE 8 Ethylene Discharge Capacity Content at −20 deg C. ElectrodeBattery (%) (mAh) Strength C1 60 102 5 70 161 3 80 170 3 95 230 2 98 2561 C2 60 105 5 70 167 4 80 185 4 95 234 2 98 268 1 C3 60  98 5 70 159 480 170 4 95 228 3 98 254 1 C4 60 102 5 70 162 4 80 180 4 95 234 2 98 2661

As shown in table 8, through the low-temperature discharge capacityincreased with increasing ethylene content, the electrode strengthdecreased conversely. Consequently, it is preferable to keep theethylene content of the ethylene-acrylic acid copolymer in the range70-95%.

Table 9 shows the relationships between the average particle size offlaky graphite and battery capacity and between the average particlesize of flaky graphite and low-temperature discharge characteristic inExample batteries C1-C4 of the present invention and Comparative batteryF. The data is for the case of a binder content of 5 parts by weightrelative to 100 parts by weight of the carbon material. Pressing wascarried out at 25 degrees C.

TABLE 9 Average Particle Size of Battery Discharge Capacity at FlakyGraphite Capacity −20 degC Battery (μm) (mAh) (mAh) C1 1 863 218 5 914200 20 922 170 30 932 155 40 944 71 C2 1 867 230 5 921 205 20 930 185 30934 159 40 941 75 C3 1 866 211 5 921 195 20 933 170 30 935 158 40 944 69C4 1 866 233 5 922 208 20 933 180 30 938 155 40 946 72 F 1 859 79 5 91361 20 935 44 30 938 20 40 945 3

As can be seen from Table 9, when the average particle size of the flakygraphite is smaller than 5 μm, the battery capacity decreasessignificantly as the irreversible capacity of the negative electrodecarbon material increases, and when greater than 30 μm, thelow-temperature discharge characteristic declines, suggesting that anaverage particle size range of 5-30 μm of the flaky graphite ispreferable.

Table 10 shows the relationships between the binder content in parts byweight in the negative electrode relative to 100 parts by weight of thecarbon material and the low-temperature discharge characteristic andbetween the binder content in parts by weight of the negative electroderelative to 100 parts by weight of the carbon material and the electrodestrength of Example batteries C1-C4 of the present invention andComparative Example battery F. The data is for the case of an averageflaky graphite particle size of 20 μm. Pressing was carried out at 25degrees C.

TABLE 10 Discharge Capacity at −20 degC Battery Binder Content (mAh)Electrode Strength C1 0.5 198 2 5 170 3 8 158 5 10 93 8 C2 0.5 210 3 5185 4 8 168 7 10 100 10 C3 0.5 201 3 5 170 4 8 160 7 10 98 10 C4 0.5 2053 5 180 4 8 164 6 10 97 9 F 0.5 58 =0 5 44 1 8 19 2 10 2 2

As can be seen from Table 10, when the binder content in parts by weightrelative to 100 parts by weight of the carbon material was larger than8, a significant decline in the low-temperature discharge characteristicwas observed, and at 0.5 there was a decrease in the electrode strength.Therefore, the preferable range of the binder content in parts by weightrelative to 100 parts by weight of the carbon material is 0.5-8.

Now, with regard to the temperature of heat treatment of the negativeelectrode after pressing, enough electrode strength is not obtained ator below the melting point of the negative electrode binder as thebinder does not melt, and the electrode strength decreases at or abovethe decomposition temperature of the binder as the binder decomposes.Therefore, an electrode with a superior electrode strength can beobtained by heat treatment at a temperature between the melting pointand decomposition temperature of the binder. Same thing applies to thepressing temperature of the negative electrode.

Though a description has been made of use of one type of binder in eachExample of the present invention, it is obvious that similar result willbe obtained by using a mixture of two or more types. It is also obviousthat similar result will be obtained when the binder is used blendedwith polyethylene and polypropylene.

Table 11 shows the low-temperature discharge characteristic, electrodestrength, and charge-discharge cycle characteristic of the Examplebatteries D1-D4 of the present invention and Comparative Battery F. Thedata is for the case of an average flaky graphite particle size of 20 μmand the binder content of 5 parts by weight relative to 100 parts byweight of the carbon material.

TABLE 11 Battery D1 D2 D3 D4 F Rolling Temperature (deg C.) 25 130 25130 25 130 25 130 25 130 Discharge Capacity at −20 deg C. (mAh) 173 208187 224 175 200 186 222 44 47 Electrode Strength 4 4 4 5 4 4 4 5 1 1Number of Cycles (cycles) 535 540 527 540 531 529 537 547 447 430

As shown in Table 11, all of the Example batteries D1-D4 of the presentinvention exhibited a characteristic superior to Comparative Examplebattery F in the low-temperature discharge characteristic. This isconsidered to be due to a lower degree of coverage of the carbonparticles with the binder compared with Comparative Example battery F.

With regard to the electrode strength, too, all of Example batteriesD1-D4 of the present invention were superior to Comparative Examplebattery F. Furthermore, in the case pressing was performed at 130degrees C., a negative electrode with a further superior electrodestrength was obtained because the negative binder of the presentinvention melts during pressing and solidifies again under the state ofbeing pressed thus enhancing the binding property. Also, the reason whythe negative electrode of Example batteries D1-D4 of the presentinvention showed especially high values of strength is considered to bedue to the fact that the negative electrode has a highly polar radical,—COOH or —COOCH₃. Furthermore, it was confirmed that when a part or thewhole of —COOH or —COOCH₃ radicals is substituted with —COO⁻Na⁺, K⁺ tomake acrylate and methacrylate, adhesiveness with the core material isfurther enhanced.

Example batteries of the present invention also showed acharge-discharge cycle characteristic which is superior to theComparative Example battery F. This is considered to be due to the factthat the binder of these batteries does not have double bonds in theprimary chain of the polymer and is chemically less reactive to liquidelectrolyte thus superior in resistance to liquid electrolyte comparedwith styrene-butadiene copolymer of the binder used in ComparativeExample battery F.

Table 12 shows the relationships between the average particle size offlaky graphite and battery capacity and between the average particlesize of flaky graphite and low-temperature discharge characteristic inExample batteries D1-D4 of the present invention and Comparative Examplebattery F. The data is for the case of a binder content of 5 parts byweight relative to 100 parts by weight of the carbon material. Pressingwas carried out at 25 degrees C.

TABLE 12 Average Particle Size of Battery Discharge Capacity at FlakyGraphite Capacity −20 degC Battery (μm) (mAh) (mAh) D1 1 863 207 5 920195 20 930 173 30 936 141 40 946 70 D2 1 861 217 5 919 202 20 929 187 30932 154 40 940 80 D3 1 868 209 5 921 196 20 938 175 30 940 147 40 945 75D4 1 870 220 5 922 204 20 937 186 30 942 155 40 948 79 F 1 859 79 5 91361 20 935 44 30 938 20 40 945 3

As can be seen from Table 12, when the average particle size of theflaky graphite is smaller than 5 μm, the battery capacity decreasessignificantly as the irreversible capacity of the negative electrodecarbon material increases, and when greater than 30 μm, thelow-temperature discharge characteristic declines suggesting that anaverage particle size range of 5-30 μm of the flaky graphite ispreferable.

Table 13 shows the relationships between the binder content in parts byweight relative to 100 parts by weight of the carbon material of thenegative electrode and the low-temperature discharge characteristic andbetween the binder content in parts by weight relative to 100 parts byweight of the carbon material of the negative electrode and theelectrode strength of Example batteries D1-D4 of the present inventionand Comparative Example battery F. The data is for the case of anaverage particle size of flaky graphite of 20 μm. Pressing was carriedout at 25 degrees C.

TABLE 13 Discharge Capacity at −20 degC Battery Binder Content (mAh)Electrode Strength D1 0.5 206 3 5 173 4 8 162 7 10 99 9 D2 0.5 219 3 5187 4 8 171 7 10 105 9 D3 0.5 207 3 5 175 4 8 166 8 10 100 10 D4 0.5 2203 5 186 4 8 170 8 10 103 10 F 0.5 58 =0 5 44 1 8 19 2 10 2 2

As can be seen from Table 13, when the binder content in parts by weightrelative to 100 parts by weight of the carbon material was greater than8, a significant decline in the low-temperature discharge characteristicwas observed, and at 0.5 there was a decrease in the electrode strength,suggesting that the preferable range of the ratio between the carbonmaterial and the binder is 0.5-8 parts by weight relative to 100 partsby weight of the carbon material.

Now, with regard to the temperature of heat treatment of the negativeelectrode after pressing, enough electrode strength is not obtained ator below the melting point of the negative electrode binder as thebinder does not melt, and the electrode strength decreases at or abovethe decomposition temperature of the binder as the binder decomposes.Therefore, an electrode with a superior electrode strength can beobtained by heat treatment at a temperature between the melting pointand decomposition temperature of the binder. Same thing applies to thepressing temperature of the negative electrode.

Though a description has been made of use of one type of binder in eachExample of the present invention, it is obvious that similar result willbe obtained by using a mixture of two or more types. It is also obviousthat similar result will be obtained by using the binder blended withpolyethylene and polypropylene.

Table 14 shows the low-temperature discharge characteristic, electrodestrength, and charge-discharge cycle characteristic of the Examplebatteries E1-E8 of the present invention and Comparative Battery F. Thedata is for the case of an average flaky graphite particle size of 20 μmand the binder content of 5 parts by weight relative to 100 parts byweight of the carbon material.

TABLE 14 Battery E1 E2 E3 E4 E5 E6 E7 E8 F Rolling Temperature 25 130 25130 25 130 25 130 25 130 25 130 25 130 25 130 25 130 (deg C.) DischargeCapacity 190 227 187 214 178 203 183 212 178 205 181 218 177 209 175 21144 47 at −20 deg C. (mAh) Electrode Strength 4 5 4 4 4 5 4 4 3 4 4 5 4 53 4 1 1 Number of Cycles 341 329 335 341 322 309 339 343 317 333 321 338304 308 315 322 447 430 (cycles)

In the low-temperature discharge characteristic, all of Examplebatteries E1-E8 of the present invention exhibited a characteristicsuperior to Comparative Example battery F as shown in Table 14. This isconsidered to be due to a lower degree of coverage of the carbonparticles with the binder compared with Comparative Example battery F.

With regard to the electrode strength, too, all of Example batteriesE1-E8 of the present invention were superior to Comparative Examplebattery F. Furthermore, in the case pressing was performed at 130degrees C., a negative electrode with a further superior electrodestrength was obtained because the negative binder of the presentinvention melts during pressing and solidifies again under the state ofbeing pressed thus enhancing the binding property. Also, the reason whythe negative electrode of Example batteries E1-E8 of the presentinvention showed especially high values of strength is considered to bedue to the fact that the negative electrode has a highly polar radical,—COOH or —COOCH₃. Furthermore, it was confirmed that when a part or thewhole of —COOH or —COOCH₃ radical is substituted with —COO⁻Na⁺, K⁺ tomake acrylate and methacrylate, adhesiveness with the core material isfurther enhanced.

Example batteries of the present invention showed a charge-dischargecycle characteristic which is inferior to the Comparative Examplebattery F. While the reason is not clear, it is assumed that, in view ofthe superiority of the binder in the resistance to liquid electrolyte,elasticity of the resin has decreased by copolymerization of styrenecausing a physical stress due to expansion and shrinkage of the carbonmaterial.

Table 15 shows the relationships between the average particle size offlaky graphite and battery capacity and between the average particlesize of flaky graphite and low-temperature discharge characteristic inExample batteries E1-E8 of the present invention and Comparative Examplebattery F. The data is for the case of a binder content of 5 parts byweight to 100 parts by weight of the carbon material. Pressing wascarried out at 25 degrees C.

TABLE 15 Average Particle Size of Battery Discharge Capacity at FlakyGraphite Capacity −20 degC Battery (μm) (mAh) (mAh) E1 1 874 234 5 922214 20 930 190 30 938 165 40 947 79 E2 1 873 230 5 918 209 20 927 187 30935 164 40 944 78 E3 1 872 212 5 923 199 20 936 178 30 942 159 40 945 77E4 1 875 229 5 920 205 20 930 183 30 937 163 40 948 74 E5 1 873 222 5922 203 20 931 178 30 939 159 40 948 69 E6 1 871 227 5 919 208 20 928181 30 936 169 40 946 74 E7 1 877 220 5 924 201 20 932 177 30 938 167 40947 71 E8 1 875 216 5 922 197 20 929 175 30 936 163 40 943 68 F 1 859 795 913 61 20 935 44 30 938 20 40 945 3

As can be seen from Table 15, when the average particle size of theflaky graphite is smaller than 5 μm, the battery capacity decreasessignificantly as the irreversible capacity of the negative electrodecarbon material increases, and when greater than 30 μm, thelow-temperature discharge characteristic declines, suggesting that anaverage particle size range of 5-30 μm of the flaky graphite ispreferable.

Table 16 shows the relationships between the binder content in parts byweight relative to 100 parts by weight of the carbon material of thenegative electrode and the low-temperature discharge characteristic andbetween the binder content in parts by weight relative to 100 parts byweight of the carbon material of the negative electrode and theelectrode strength of Example batteries E1-E8 of the present inventionand Comparative Example battery F. The data is for the case of anaverage flaky graphite particle size of 20 μm. Pressing was carried outat 25 degrees C.

TABLE 16 Discharge Capacity at −20 degC Battery Binder Content (mAh)Electrode Strength E1 0.5 231 3 5 190 4 8 176 6 10 104 9 E2 0.5 226 3 5187 4 8 171 6 10 100 10 E3 0.5 213 3 5 178 4 8 166 6 10 87 10 E4 0.5 2223 5 183 4 8 169 6 10 98 9 E5 0.5 216 2 5 178 3 8 164 4 10 92 8 E6 0.5221 3 5 181 4 8 168 5 10 94 9 E7 0.5 220 3 5 177 4 8 169 6 10 96 10 E80.5 211 2 5 175 3 8 162 4 10 92 7 F 0.5 58 =0 5 44 1 8 19 2 10 2 2

As can be seen from Table 16, when the binder content in parts by weightrelative to 100 parts by weight of the carbon material is larger than 8,a significant decrease in the low-temperature discharge characteristicwas observed, and at 0.5, there was a decrease in the electrodestrength. Therefore, the preferable range of the binder content in partsby weight relative to 100 parts by weight of the carbon material is0.5-8.

Now, with regard to the temperature of heat treatment of the negativeelectrode after pressing, enough electrode strength is not obtained ator below the melting point of the negative electrode binder as thebinder does not melt, and the electrode strength decreases at or abovethe decomposition temperature of the binder as the binder decomposes.Therefore, an electrode with a superior electrode strength can beobtained by heat treatment at a temperature between the melting pointand decomposition temperature of the binder. Same thing applies to thepressing temperature of the negative electrode.

Though a description has been made of use of one type of binder in eachExample of the present invention, it is obvious that similar result willbe obtained by using a mixture of two or more types. It is also obviousthat similar result will be obtained when the binder is used blendedwith polyethylene and polypropylene.

In the examples of the present invention, though flaky graphite was usedas the negative electrode carbon material, similar effects were obtainedirrespective of the type and configuration of the carbon materials.

Also, while LiCoO₂ was employed as the positive active material, similareffects were obtained by employing other positive active material suchas LiNiO₂ or LiMn₂O₄.

INDUSTRIAL APPLICATION

As has been described above, the present invention provide a negativeelectrode which is superior in the low-temperature dischargecharacteristic and in the strength against peeling of the electrode mix,and, through use of the negative electrode, it also provides anon-aqueous electrolyte secondary battery which is superior in the easeof handling in mass production, high in reliability, and superior indischarge characteristic.

What is claimed is:
 1. A non-aqueous electrolyte secondary batterycomprising a rechargeable positive electrode, a non-aqueous liquidelectrolyte, and a negative electrode comprising a negative electrodematerial; in which: the negative electrode material comprises (1) acarbon material that is capable of absorbing and desorbing lithium and(2) a binder; the carbon material is a graphite material; the binder isat least one material selected from the group consisting ofethylene-acrylic acid copolymers, ethylene-acrylate copolymers,ethylene-methyl acrylic acid copolymers, ethylene-methacrylic acidcopolymers, ethylene-methacrylate copolymers, and ethylene-methylmethacrylic acid copolymers; and; the ethylene content of the binder isin the range of 70-95%.
 2. The non-aqueous electrolyte secondary batteryof claim 1 wherein said carbon material is a graphite material having anaverage particle size in the range 5-30 μm.
 3. The non-aqueouselectrolyte secondary battery of claim 1 wherein the ratio between saidcarbon material an said binder is such that the binder content is in therange of 0.5-8 parts by weight relative to 100 parts by weight of thecarbon material.
 4. The non-aqueous electrolyte secondary battery ofclaim 3 wherein said carbon material is a graphite material having anaverage particle size in the range 5-30 μm.
 5. The non-aqueouselectrolyte secondary battery of claim 4 wherein the binder is at leastone material selected from the group consisting of ethylene-acrylic acidcopolymers, ethylene-methyl acrylic acid copolymers,ethylene-methacrylic acid copolymers, and ethylene-methyl methacrylicacid copolymers.
 6. The non-aqueous electrolyte secondary battery ofclaim 1 wherein the binder is at least one material selected from thegroup consisting of ethylene-acrylic acid copolymers, ethylene-methylacrylic acid copolymers, ethylene-methacrylic acid copolymers, andethylene-methyl methacrylic acid copolymers.
 7. The non-aqueouselectrolyte secondary battery of claim 2 wherein the binder is at leastone material selected from the group consisting of ethylene-acrylic acidcopolymers, ethylene-methyl acrylic acid copolymers,ethylene-methacrylic acid copolymers, and ethylene-methyl methacrylicacid copolymers.
 8. The non-aqueous electrolyte secondary battery ofclaim 1 wherein the rechargeable positive electrode comprises alithiated transition metal oxide as a positive active material.
 9. Thenon-aqueous electrolyte secondary battery of claim 4 wherein therechargeable positive electrode comprises a lithiated transition metaloxide as a positive active material selected.
 10. The non-aqueouselectrolyte secondary battery of claim 2 wherein the binder comprises—COO⁻Na⁺ or —COO⁻K⁺ groups.
 11. The non-aqueous electrolyte secondarybattery of claim 3 wherein the binder comprises —COO⁻Na⁺ or —COO⁻ K⁺groups.
 12. The non-aqueous electrolyte secondary battery of claim 11wherein the binder comprises —COO⁻Na⁺ or —COO⁻K⁺ groups.
 13. Thenon-aqueous electrolyte secondary battery of claim 4 wherein the bindercomprises —COO⁻Na⁺ or —COO⁻K⁺ groups.
 14. The non-aqueous electrolytesecondary battery of claim 8 wherein the positive active material isselected from the group consisting of LiCoO₂, LiNiO₂, and LiMn₂O₄. 15.The non-aqueous electrolyte secondary battery of claim 9 wherein thepositive active material is selected from the group consisting ofLiCoO₂, LiNiO₂, and LiMn₂O₄.
 16. The non-aqueous electrolyte secondarybattery of claim 4 wherein the binder is blended with polyethylene orpolypropylene.
 17. The non-aqueous electrolyte secondary battery ofclaim 1 wherein the binder is blended with polyethylene orpolypropylene.
 18. A method of manufacturing a non-aqueous electrolytesecondary battery negative electrode comprising a negative electrodematerial, the negative electrode material comprising a carbon materialwhich is capable of absorbing and desorbing lithium and a binder;wherein said carbon material is a graphite material, said binder is atleast one material selected from the group consisting ofethylene-acrylic acid copolymers, ethylene-acrylate copolymers,ethylene-methyl acrylic acid copolymers, ethylene-methacrylic acidcopolymers, ethylene-methacrylate copolymers, and ethylene-methylmethacrylic acid copolymers and, the ethylene content of said binder isin the range of 70-95%; the method comprising the steps of: coating amixture of said carbon material and said binder on a current collector,drying and pressing said mixture, and heat treating said mixture at atemperature between the melting point and the decomposition temperatureof said binder, or pressing said mixture at a temperature between themelting point and the decomposition temperature of said binder.
 19. Themethod of claim 18 wherein the ratio between said carbon material andsaid binder is such that the binder content is in the range of 0.5-8parts by weight relative to 100 parts by weight of the carbon material.20. The method of claim 18 wherein said carbon material is a graphitematerial having an average particle size in the range 5-30 μm.
 21. Themethod of claim 19 wherein said carbon material is a graphite materialhaving an average particle size in the range 5-30 μm.
 22. The method ofclaim 21 wherein the binder comprises —COO⁻ Na⁺ or —COO⁻ K⁺ groups.